An RF generator that converts the DC of a DC power supply to RF AC through the switching operation is known. As this RF generator, a class-D RF generator using a class-D amplifier circuit (Class D: IEC International Standard IEC 60268-3 4 classes of operation) is known.
A class-D RF generator causes the RF power amplifier device to perform the switching operation via the RF gate signal, which has a predetermined duty ratio, to convert the DC of a DC power supply to RF AC and supplies the obtained RF AC to a load as RF forward wave power. The class-D RF generator adjusts the output through the pulse operation. The pulse operation is performed in a driving mode in which ON state and the OFF state are alternated by the RF gate signal. In the ON state, the switching operation is performed by the RF power amplifier device and RF power is output; in the OFF state, the switching operation is not performed and RF power is not output. The RF output power is adjusted by changing the duty ratio that is a time ratio between the ON state and the OFF state. The duty ratio between the ON state and the OFF state can be controlled by a duty ratio between the ON state and the OFF state of the pulse control signal. In this specification, RF means high frequency.
When RF power is supplied from an RF power generator to a load, for example, to a load such as a plasma treatment device, the load impedance varies according to the state of a plasma discharge. When the load impedance varies, reflected wave power that returns from the load side to the RF generator side varies.
In some cases, reflected wave power affects a class-D RF generator. For example, an RF power amplifier device, which is one of the components of a class-D RF generator, may suffer thermal damage due to heat generated by an internal loss caused by reflected wave power or may suffer insulation damage due to a surge voltage of reflected wave power. A still larger reflected wave power sometimes damages the DC power supply that is one of the components of a class-D RF generator.
In particular, when a class-D RF generator supplies RF power to a plasma load through the pulse operation, forward wave power all returns to the generator side as reflected wave power in the non-ignition state in which plasma is not yet ignited. Therefore, a class-D RF generator is required to tolerate total reflected wave power. In the description below, reflected wave power generated when forward wave power all returns to the generator side is called total reflected wave power, and the ability to tolerate total reflected wave power during RF power supply is called total reflected wave power tolerance.
The total reflected wave power tolerance includes the ability not only to prevent an RF power amplifier device from being damaged by total reflected wave power but also to continue the plasma ignition operation by continuously supplying power without interruption from the time the ignition operation is started to the time the supply of RF power is stopped upon determination that the ignition has failed.
Conventionally, to realize such total reflected wave power tolerance, the present applicant has proposed a plasma generation power supply (see Patent Literature 1). Patent Literature 1 describes a technology that uses load impedance, viewed from an RF power amplifier device, in the delay state in order to suppress accumulated carriers in the body diode of the RF power amplifier device and thereby reduces the switching loss of the circuit. To realize total reflected wave power tolerance, it is further required to limit the ignition time, necessary for the ignition operation at plasma ignition time, in advance and, by doing so, to limit the reflected wave power within this ignition time to the same power level as that of the rated power of forward wave power.
Not only a class-D RF generator, a class-C RF generator is also known as a RF generator that is usually used. For use by a usually used RF generator device such as a class-C RF generator, a technology is known that reduces forward wave power on the supplying side in order to reduce the supply of reflected wave power to a level equal to or lower than the rated output when reflected wave power is generated. In this way, the technology prevents the device on the RF generator side from being damaged (Patent Literature 2 to Patent Literature 7).
Patent Literatures 2 and 3 disclose technologies for stopping the supply of power, and Patent Literatures 4 to 7 disclose technologies for reducing forward wave power.
Patent literature 2 describes a shutdown method for controlling the forward wave power value of an RF plasma power supply so that the reflected wave power value becomes equal to or lower than 10% to 20% of the rated output. Patent literature 3 describes a microwave power supply system that uses a signal, output from a reflected wave power detector, to temporally integrate the differences between the magnitude of the signal, corresponding to the reflected microwave power, and the charge/discharge reference value and, if the magnitude of the integrated signal having the magnitude corresponding to the integrated value exceeds the allowable reference value, shuts down the supply of power.
Patent literature 4 describes a technology that reduces the output power using a mixer when reflected wave power exceeds the limit value. Patent literature 5 describes a technology that generates a power control signal using the power reduction signal, output from the reflected wave power detection signal, and forward wave power. Patent literature 6 describes a technology that calculates the difference between the reflected wave power, which is detected and fed back, and the reflected wave power that is set and, based on the calculated difference, drops the forward wave power. Patent literature 7 describes a technology that calculates a reflected wave coefficient based on the reflected wave power, corrects the magnitude of the gain of the attenuator according to the calculated reflected wave coefficient, and supplies required power to the load.
Patent literature 8 describes a technology that differentiates the output of the sensor, which measures reflected wave power, and determines the generation of an abnormal discharge based on the degree of temporal variations in the reflected wave of RF power output by the differentiation.